It is generally desirable to dispose balance weights of a balancer in a reciprocating piston engine within a plane that intersects the center of a crank shaft at right angles with its axis, with their shafts being supported by a diaphragm or cylinder-partition portion, having high rigidity, of a cylinder block between cylinders. However, it is usually impossible to dispose the balance weights within such plane intersecting the center of the crank shaft, because said diaphragm of the cylinder block is placed therein.
In order to make it clearer, a conventional balancer of this type in a two-cylinder engine will be illustrated with reference to FIG. 1A. Journals 2' and 3' at both ends of a crank shaft 1' are supported by bearings 6' and 7' in front and rear walls 4' and 5' of a cylinder block, respectively. Meanwhile, a journal 8'" at the center of said crank shaft is supported by a bearing 12' in a diaphragm portion 11' of the cylinder block interposed between a first cylinder 9' and a second cylinder 10'. Therefore, in order to dispose balance weights 19' and 20' near the center of said crank shaft, balancer shafts 13' and 14' must be supported by bearings 15' and 16' provided in the front wall 4' and bearings 17' and 18' in the diaphragm 11' of said cylinder block, respectively. Consequently, the balance weights 19' and 20' are cantilevered as illustrated in FIG. 1A.
When the engine of this type in which the balance weights 19' and 20' are cantilevered is used for a long time, only one side of said balance weights become worn down as a result of coming in contact with the bearings 17' and 18'. This has caused vibrations, which in turn has produced cracks in the cylinder block and thereby shortened the life of the engine.
In an arrangement shown in FIG. 1B, a balance weight of the above-described type is not cantilevered. That is, balancer shaft journals 22' and 23' at both ends of a balance weight portion 21' are supported by bearings 24' and 25' provided in the cylinder block. A disadvantage of this arrangement is that provision of an additional bearing results in an increase in cost. Besides, as in the case of the above-described conventional system shown in FIG. 1A, the balance weight cannot be disposed along the plane passing through the center of the crank shaft, occupied by the diaphragm portion 11' of the cylinder block. Furthermore, the use of the two bearings 24' and 25' makes it impossible to place them in the diaphragm portion of the cylinder block that has high rigidity. Consequently, at least one of them has to be placed in a less rigid portion of the cylinder block. This has caused vibration, resonance and noises.
In addition, even in the case of FIG. 1B, the facing sides of the bearings 24' and 25' have seized as a result of coming in contact with the balance weight when the balancer shaft rotated at high speed.